1. Technical field
This invention relates to the field of tape guides for a reel-to-reel tape drive system.
2. Background art
Data processing systems have traditionally utilized magnetic tape as a data storage medium. Information is stored on magnetic tape in a very dense format. The position of the magnetic tape must be precisely controlled with respect to a read/write magnetic head to provide accurate reading and writing of information on the magnetic tape.
Typically, magnetic tape is wound and rewound between reels in a reel-to-reel tape drive. A prior art reel-to-reel tape drive system is illustrated in FIG. 1a. The tape drive includes supply reel 1 on which tape 7 is initially wound, first tape guide 2, second tape guide 3a, magnetic read/write head 4, third tape guide 3b, fourth tape guide/transducer 5, and take up reel 6. One critical area for accurately positioning magnetic tape 7 is in the vicinity of the read/write magnetic head 4. Read/write magnetic head 4 reads information from and writes information to magnetic tape 7. Tape guides 3a and 3b are placed just prior to and just after read/write magnetic head 4 to ensure the appropriate tracks of magnetic tape 7 are positioned relative to magnetic head 4.
A second critical area for accurately controlling the position of the magnetic tape is immediately following the supply reel and immediately preceding the take-up reel. Referring again to FIG. 1a, first tape guide 2 decouples the tape from supply reel 1 and fourth tape guide 5 guides the tape onto the take-up reel 6. Note that reel-to-reel tape drives are bi-directional. Therefore, at times when the direction in which the tape is processed through the drive is reversed, supply reel 1 acts as a take-up reel and first tape guide 2 will guide the tape onto the supply reel 1. Similarly, take-up reel 6 behaves as a supply reel and fourth guide 5 performs as a fixed guide.
The position of the magnetic tape edge as it leaves the supply reel can be vertically displaced by as much as .+-.0.050 inch from its nominal tape edge location at the read/write head. Of the .+-.0.050 inch position error, .+-.0,020 inch arises from manufacturing tolerances associated with the vertical location of the supply reel with respect to the tape path or read/write head. The remaining .+-.0.030 inch is a result of "stagger wrap." A cross sectional view of a tape reel with stagger wraps 91 is illustrated in FIG. 1b. Stagger wraps 91 are caused by the vertical wandering of the tape as it is being wound onto a reel--possibly from one flange 90 of the reel to the other. Stagger wraps 91 are wraps of the tape around the tape reel, stacked against either the top or bottom of the supply reel flanges 90.
Vertical position error of the tape also occurs when the tape, as it is processed in the reverse direction, exits the take-up reel. Because the take-up reel is fixed, the overall vertical position error can be kept smaller, on the order of .+-.0.020 inch.
Modern reel-to-reel tape drives use two types of tape guides to control the positioning of magnetic tapes. A first type, commonly referred to as a fixed guide, is illustrated in FIG. 2. The fixed guide is comprised of fixed flanges 10a and 10b which are affixed to the top and bottom ends of the arcuate surface 12. Both flanges 10a and 10b help guide magnetic tape 11 over arcuate surface 12. Without flanges 10a and 10b, magnetic tape 11 might become displaced with respect to arcuate surface 12 and could, under appropriate circumstances, allow magnetic tape 11 to move away from the read/write head so that information could not be read from or written to the magnetic tape.
A second type of tape guide, commonly referred to as a fine guide, is illustrated in FIG. 3. The fine guide is comprised of one fixed flange 20, one compliant flange 23 and arcuate surface 25. Fixed flange 20 is affixed to the bottom end of arcuate surface 25 and compliant flange 23 is affixed to the top end. Compliant flange 23 forces magnetic tape 24 against fixed flange 20. In one particular prior art embodiment of a fine guide, illustrated in FIG. 3, compliant flange 23 is comprised of a plurality of spring members 22 extending individually from a base member. At the end of each spring member 22, a pad 21 having high wear resistance and high mechanical strength is mounted. The separate pads 21 individually contact tape 24, thus ensuring the performance of the guide as a true compliant guide.
There are a number of disadvantages to using either fixed guides or fine guides in reel-to-reel tape drives. One disadvantage associated with fixed guides is the result of stagger wraps. Stagger wraps, as described above, are the vertical displacement of magnetic tape 7 against one or the other of the reel flanges 90 (see FIG. 1b).
Referring back to FIG. 1a, stagger wraps may be present when the tape leaves supply reel 1 and enters first tape guide 2. In present tape drives, there is approximately 2.5 inches between supply reel 1 and the first tape guide 2. The tape may be displaced up to .+-.0.050 inch from its nominal position when it exits reel 1. If the tape exits reel 1 up 0.050 inch, the tape will be forced against the upper flange of first tape guide 2. Likewise, if the tape exits the reel 1 down 0.050 inch, the tape will be forced against the lower flange of first tape guide 2. With no ability to correct totally for this displacement, tape 7 can buckle and crease FIG. 4, potentially causing a loss of data. At a minimum, it causes poor tracking of magnetic tape 7 and dramatically increases tape wear.
Another problem associated with fixed guides relates to the friction generated from the movement of the magnetic tape against the fixed flanges of such guides. Magnetic tape is often manufactured from chromium dioxide. The fixed flanges of the fixed guides are generally composed of stainless steel, a softer substance than chromium dioxide. The movement of the abrasive edge of the chromium dioxide magnetic tape against the tape guides erodes the softer stainless steel fixed flanges. This erosion causes grooves to develop in the tape guides which reduces accuracy of the positioning of the magnetic tape relative to the read/write head and transducers. Further, the grooves increase the surface friction of the tape guides which increases the rate at which magnetic tape wears out. In addition, debris accumulates as a result of the erosion which further reduces the effectiveness of the tape drive.
For the foregoing reasons, there is a need for a tape guide that (1) minimizes the effects of stagger wraps by providing gentle tape guidance forces and coarse correction for vertical displacement error of the magnetic tape; and (2) reduces the amount of debris accumulation and tape wear.